Support specifying an app's minSdk

[lberrymage]

Currently Accrescent assumes that all apps in its repository are compatible with the client OS. However, this is not necessarily the case, and this behavior will lead to hard-to-diagnose errors when installing an app that doesn't support the client Android version. The repository metadata should reflect the minimum version of Android an app supports, and Accrescent should respect this.

It's not immediately clear how best to display an app that can't be installed on a user's device. We could display it in search results but disable the install button and display a message explaining that the app doesn't support the user's device, or we may even want to hide it from displaying in the search results altogether. Google Play seems to take the latter approach, and handles deep links to an app's details screen with the former behavior.